1. Field of the Invention
The present invention relates to a superplastic alloy-containing conductive plastic article for shielding against electromagnetic interference (EMI), and more particularly relates to a process for manufacturing a conductive plastic article containing one continuous superplastic alloy layer in one processing stage.
2. Description of the Prior Art
In recent years, progress in technology has led to an extensive increase in the amount of sophisticated electronic equipment. However, the high-density electromagnetic waves produced from electronic equipment have the potential to damage or adversely affect the performance of other equipment or components. Also, exposure to electromagnetic waves is harmful to the human body. Therefore, an electrically conductive outer shell is needed to shield electromagnetic interference (EMI) produced from electronic equipment.
Heretofore, various methods have been used to shield electronic equipment. Metallic boxes and cans fabricated from steel, copper, aluminum, etc., were used to surround high EMI emitters as shielding. However, metallic shields with intricate shapes were difficult to be fabricated by the conventional metalworking methods. Moreover, metallic shields were cumbersome, heavy and costly. Therefore, the electronic industry has resorted to metallized plating on plastics. Unfortunately, the results obtained with metallic coatings were not always satisfactory. In addition to being relatively non-economical, once such metallic coatings were scratched through, they would lose part of their shielding efficiency. Unless such conductive coatings are continuous and free of voids, electromagnetic waves will be free to pass through. Frequently, it was difficult to obtain a dependable, 100% effective coating which was also resistant to peeling.
Further efforts by the electronics industry to develop more dependable light-weight materials for EMI shielding have led to a third approach, namely electrically conductive component-filled plastic composites. It was anticipated that intricate shapes could be molded from the composite materials by conventional means, yielding a finished part that promised to be more economic and dependable than metal or metal-coated plastics.
The principle factor influencing the performance of conductive component-filled plastic composites is the aspect ratio of the conductive fillers. The aspect ratio is defined as the ratio of the maximum dimension to the minimum dimension of the filler. For example, the aspect ratio of a fiber is the ratio of the length to the diameter of the fiber. According to the electromagnetic wave percolation theory, if the conductive filler in the plastic retains a higher aspect ratio, the filler easily forms a conductive network, thus, the critical concentration of the conductive filler required to achieve the electromagnetic shielding effect (that is, the threshold percolation concentration) is lower.
The methods for preparing conductive component-filled plastic composites can be classified into four types. The first type involves compounding the conductive fillers in the form of powders, short fibers or flakes with the plastic matrix, and then having the mixture hot-press molded or injection molded into various kinds of plastic products for shielding EMI.
For example, U.S. Pat. No. 4,474,685 discloses a process for fabricating electromagnetic shielding products by first compounding and then molding a composition including a thermosetting resin binder and an electrically conductive filler (including carbon black, graphite and conductive metal powders). However, during the compounding with the resin matrix, the conductive powders may easily cluster, and thus are not capable of dispersing in the resin matrix. Consequently, the electromagnetic shielding efficiency of the molded products can not be effectively improved. Furthermore, since the powder filler has a lower aspect ratio, according to the electromagnetic wave percolation theory as mentioned above, the amount (i.e., threshold percolation concentration) of the powder filler added must be relatively high to achieve electrical conductivity. Consequently, the mechanical properties, color and other physical and chemical properties of the molded products are adversely affected.
On the other hand, if the conductive filler is in a higher aspect ratio form such as fibers or flakes, the filler can be loaded to a lower level. However, the cluster phenomenon is still difficult to prevent. In addition, during the compounding process, in order to maintain the original aspect ratio, the conductive filler should be strong enough to prevent breakage due to compounding. However, such a strong conductive filler is very expensive, and is thus not suitable for ordinary low cost electronic equipment.
The second type of method for preparing conductive component-filled plastic composites involves binding a plastic layer to enclose the conductive continuous filler by immersion or extrusion, and then cutting the conductive long fiber-filled plastic stick to a predetermined length. For example, Japanese Patent No. 60-112854 discloses a process including continuous extruding thermoplastic plastic to enclose a copper fiber to form a copper fiber-filled plastic round stick, and then cutting the plastic round stick into pellets of a predetermined size. In order to increase the aspect ratio of the filler, the diameter of the conductive long fiber should be as small as possible. The fibrous filler must be strong enough to prevent breakage, but suitable fillers, such as stainless steel fiber, copper fiber or metal-coated carbon fiber, are very expensive.
To decrease the total cost of the production of conductive component-filled plastic composites, aluminum filler, which has the advantages of low price, low density, excellent electromagnetic shielding efficiency, and easy of color matching, has already been used. However, since aluminum has low strength, when aluminum material is utilized in the first method for preparing aluminum-filled plastic composites, the process involves compounding aluminum flakes with plastic. However, since aluminum has low strength, many aluminum flakes or fibers break during processing, resulting in a rapid decrease of the aspect ratio. Therefore, the incorporation amount (threshold percolation concentration) should be increased to a very high level (generally, as high as 30 to 40%) to achieve an acceptable electromagnetic shielding efficiency. The consequence is that the total cost is increased, and more seriously, the electromagnetic shielding plastic products obtained have poor mechanical properties. For example, elongation, tensile strength, bending strength and impact strength are all adversely affected. Also for the second method, the low strength of aluminum causes the rupture of continuous long fibers, which results in an interruption of the binding process.
In order to solve the above-mentioned problems, the inventor of the present invention with his coworker have disclosed a third type of process making metallized plastic pellets in U.S. Pat. No. 5,531,851, in which radially arrangedmetal is filled. The process involves sandwiching an electrically conductive metal foil in between two plastic films to form a metallized laminated plastic sheet; slicing the plastic sheet into plastic strips; radially arranging the metallized plastic strips into a die of an extruder to be wetted and bound by molten plastic into a metallized plastic bar; and finally cutting the plastic bar into metallized plastic pellets of a predetermined size.
In the third method, aluminum can be successfully filled in the plastic. In addition, since no conventional compounding step is needed, the breakage of aluminum can be prevented and a higher aspect ratio can be maintained. Also due to the employment of a wider material (aluminum foil) with is further reinforced with plastic, the interruption of binding process in the second method will not occur. However, the procedures and apparatus for manufacturing such radially arranged metal filled pellets are very complicated, and the resulting pellets must be subjected to hot-press molding or injection molding to obtain the final plastic product for shielding electromagnetic interference. The total apparatuses required are very expensive.
The above three methods involve subjecting the conductive pellets to hot-pressing or injection molding to obtain the final plastic article for shielding EMI. When injection molding is employed, in order to prevent lag and orientional phenomena from occurring to the metallic filler in the mold, the design of the mold is very critical and complicated, thus increasing costs. Moreover, if a very thin product is desired, such as the outer shell of a notebook computer, the design of the mold is difficult, or even impossible to achieve.
In addition, floating phenomenon in the final conductive plastic article can not be prevented. Therefore, surface coating on the final conductive plastic article is required, thereby increasing cost. Most importantly, according to the above three methods, the conductive filler in the plastic is not continuous. Therefore, the resulting electromagnetic shielding effectiveness is limited, and is far less than that of a conventional metal plate or a plastic plate coated with conductive metal.
Generally speaking, the shielding effectiveness of a conventional metal plate or a plastic plate coated with metal can reach 60 dB or more, which meets the requirement for various electric equipment in various countries (FCC for U.S.A. , VDE for Germany, VCCI for Japan, CSA for Canada, CISPR for Russia) . However, a plastic article containing discontinuous conductive filler obtained from the above three methods has a shielding effectiveness of only about 40 dB, which can meet the requirements of general personal computer or other simple electric equipment.
Nowadays, personal computers are usually equipped with CD-ROM, LSI, or other circuits of high density and high frequency. Therefore, in the future, it is anticipated that personal computers will require a higher shielding effectiveness, for example, 50 dB or more. Obviously, the plastic articles obtained from the above three methods will not meet the future needs.
The fourth type of method for preparing conductive component-filled plastic composites applies vacuum molding technique. Japanese Patent No. 61-293827 discloses such method, which involves introducing the molten lead/tin alloy having a low melting point into the space between two molten plastic layers; rolling it to form a plastic/metal/plastic laminated plate; and subjecting the plate to vacuum molding at a temperature higher than the melting point of the metal and the softening point of the plastic to form a predetermined shape. By employing this method, hot-pressed molding and injection molding can be omitted, and production costs can be reduced. The conductive filler inside the plastic is in a continuous form.
However, during the process for manufacturing the plastic/metal/plastic laminated plate in the fourth method, the molten metal and the plastic have different viscosity and strength. Therefore, the thickness ratio of the metal to plastic in the laminated plate is very difficult to control. More seriously, during the vacuum molding process, the metal is in the molten state. Thus it has no bonding to the two plastic plates, and thereby, separation of the laminated plate very easily occurs. Even if the separation does not occur, when the laminated plate is attached to the wall of the mold, the metal is in the molten state and is flowing. Thus, the metal liquid will accumulate at slots, holes or other concave sites of the mold, and the resulting product for shielding EMI will have defects and is even strongly distorted.
In addition, the metal used in the fourth method has a melting point lower than the softening point of the plastic. This indicates that the final product can only be applied to a temperature lower than the melting point of the metal. Therefore, when the electronic elements including the product obtained from the fourth method are operated and generate heat at some points, the plastic outer shell and the electronic elements will be destroyed in those areas, causing very severe adverse results.